Nowadays, as electronic product designs reveal a trend towards light weight, thinness and compactness, the optical shading problem in an image pickup apparatus has become more and more severe. Optical shading refers to an angular variation of a light illuminating through a lens on an image sensor which causes a smaller reading from a corner region of the image sensor than from a central region. In a CMOS image sensor adapted for a fixed-focus lens, for example, as the fixed-focus lens gets closer to the image sensor, an included angle formed by a corner and a normal of the center of the image sensor increases, and accordingly the optical shading phenomenon becomes more severe.
The optical shading phenomenon is directly proportional, approximately, to the fourth power of the cosine (cos4 θ) of the included angle formed by the image sensor normal and a line connecting the pixels on the image sensor and the center of the lens. Due to the nonlinearity of this function, compensation using circuit frameworks employing conventional column analog-to-digital converters (ADCs) or chip level ADCs is difficult. Nevertheless, after the data outputted from the image sensor has been converted into a digital image, back-end digital signal processing (DSP) may be employed to mitigate the optical shading problem, for example by searching a look-up table for the compensating gain of each pixel position. However, when the pixel quantity or the image refresh rate increase, the impact of an image output due to the lengthy signal processing time needed for the massive amount of calculations involved may happen.